Flat tubes are known from a number of older applications. One of the applications is German Patent App. No. DE 10 2006 006 670.7. FIG. 1 of German Patent App. No. DE 10 2006 006 670.7 shows a flat tube in cross section. This flat tube has a corrugated inner part z in addition to the features enumerated above. The flat tube is defined by an extremely thin wall thickness, for example in the range from 0.03 mm-0.15 mm or slightly above. Such flat tubes have already been manufactured by the applicant and installed on a trial basis in heat exchangers. Until now, such heat exchangers have successfully passed all the validation phases so that they are expected to be made commercially available soon.
In some flat tubes, undesired changes in shape of the not yet brazed flat tubes occurs within the course of manufacture. The flat tubes are brazed later, after they have been pre-mounted in the heat exchanger core using fins. The changes in flat tube shape are possibly due to stresses present in the material (e.g., aluminum or steel in sheet form), or are generated by the necessary shaping of the strips. The changes in shape can lead to unsatisfactory braze connections, but they can also, for example, impede the joining to the corrugated inner part z (see FIG. 1). FIGS. 2a and 2b of the present application are intended to clarify the problem referred to here. One of the narrow sides of the flat tube is illustrated in the left of FIG. 2a. From the two illustrations on the right hand side of FIG. 2a, it is apparent that the curves in the narrow sides of the flat tube can spring apart from one another as a result of the effect of the shaping force during manufacture of the flat tubes. As a result, and as stated above, the brazing quality in the subsequent brazing process suffers. The arrows in FIG. 2a are intended to illustrate the forces which can lead to the aforementioned springing apart, and which act on the flat tube in the course of its processing to form the heat exchanger. One of the narrow sides of the flat tube is also illustrated in FIG. 2b. From the illustration on the right hand side of FIG. 2b it is apparent that, as a result of unequal application of force during the shaping process of the flat tubes, the relatively large curve does not remain in its provided position as a result of internal stresses.
In the already mentioned FIG. 1, it is also apparent that the strips or the wall parts of the flat tube can be displaced in the directions of the arrows shown there, which ultimately could cause the strips to fall apart if the above-mentioned inner part z were not present.